The prior art is replete with containers, especially clamshell-type containers which are often used in a fast-food environment to package hamburgers, chicken, pizza, or the like. Particularly when these containers are made from thinner grades of paperboard, opening them sometimes becomes difficult because, depending upon the dimensions of the container, the paperboard containers develop sufficient resistance to opening to create a back region which buckles. This buckling typically occurs around the center region of the back panel of the clamshell container, both above and below the hinge line which characterizes these containers. It is necessary (and awkward) for the consumer to press his hand or fingers in the region that has buckled to obtain proper opening of the package.
For this reason, the prior att has shown various ways to deal with this "buckling" problem. Typical of these is U.S. Pat. No. 4,232,816, which discloses a clamshell-type carton which on the back panel thereof and perpendicular to the hinge line, has formed . thereon adjacent slits which are normal to, and bisect, a hinge line. Such slits are intended to offer sufficient stress relief so that the back wall of the clamshell carton does not permanently buckle when opened.
Despite these efforts of the prior art, it has still not been a satisfactory state of affairs because "buckling" still occurs, especially in paperboard clamshell containers, particularly when thinner grades of paperboard are used.
Accordingly, it is a primary object of this invention to provide a novel type of stress relief in a paperboard clamshell container to assure a buckle free opening along the hinge line thereof.